Hey all,does anybody know the correct formula to figure out the right powervalve on a holley carb?I have heard that you divide your idle vacum in half.and then add.5 to that for the right #.Is this correct?My idle vacum is 12.5
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From the Holley website:
Each Holley power valve is stamped with a number to indicate its vacuum
opening point. For example, the number "65" indicates that the power valve
will open when the engine vacuum drops to 6.5" Hg, or below.
An accurate vacuum gauge, such as Holley P/N 26-501, should be used when
determining the correct power valve to use.
A competition or race engine which has installed a long duration, high overlap
cam will have low manifold vacuum at idle speeds. If the vehicle has a
manual transmission, take the vacuum reading with the engine thoroughly
warmed up and at idle. If the vehicle is equipped with an automatic
transmission, take the vacuum reading with the engine thoroughly warmed
up and idling in gear. In either case, the power valve selected should have
a vacuum opening point about 2" Hg below the intake manifold
vacuum reading taken.
A stock engine, or one that is only mildly built for street use, will have high
manifold vacuum (17" to 21" Hg) at idle speeds. To determine the correct
power valve, the vehicle should be driven at various steady speeds and
vacuum readings taken. The power valve selected should have an opening
point about 2" Hg below the lowest steady speed engine vacuum observed.
Holley has a 6.5" Hg power valve, P/N 125-65, which usually works out
well for most driving situations.
Each Holley power valve is stamped with a number to indicate its vacuum
opening point. For example, the number "65" indicates that the power valve
will open when the engine vacuum drops to 6.5" Hg, or below.
An accurate vacuum gauge, such as Holley P/N 26-501, should be used when
determining the correct power valve to use.
A competition or race engine which has installed a long duration, high overlap
cam will have low manifold vacuum at idle speeds. If the vehicle has a
manual transmission, take the vacuum reading with the engine thoroughly
warmed up and at idle. If the vehicle is equipped with an automatic
transmission, take the vacuum reading with the engine thoroughly warmed
up and idling in gear. In either case, the power valve selected should have
a vacuum opening point about 2" Hg below the intake manifold
vacuum reading taken.
A stock engine, or one that is only mildly built for street use, will have high
manifold vacuum (17" to 21" Hg) at idle speeds. To determine the correct
power valve, the vehicle should be driven at various steady speeds and
vacuum readings taken. The power valve selected should have an opening
point about 2" Hg below the lowest steady speed engine vacuum observed.
Holley has a 6.5" Hg power valve, P/N 125-65, which usually works out
well for most driving situations.
Thanks Charlie and Mark for responding.I put in 10.5 powervalve and an adjustable metering block today and the car runs so much better.No more off idle bog.It now revs crisp and clean.It is so much more fun to drive!!!!!!!!!